In the latest dirty water deal to be uncovered in California, oak trees have been found to supply water from deep underground directly to fungi in exchange for vital nutrients. Although plants have long been known to supply sugar to beneficial root fungi, this is the first proof that water is part of the bargain. The finding could help explain how plants persist in dry environments, where roots can access water but not nutrients stuck in dry surface soil.

Most plants have a mutualistic relationship with soil fungi called mycorrhizae, whose fine filaments expand root surface area to absorb more nitrogen, phosphorus, and other nutrients. To keep these fungi alive and feeling generous, plants transfer some of the sugar they make by photosynthesis into the soil. Many trees are also known to pump water from deep soil layers, which leaks out and benefits plants and fungi on the surface. But most scientists assumed that such transfer was indirect and insufficient to sustain root-penetrating fungi during drought.

Others weren't so sure. To test that assumption, ecologist Jose Querejeta and colleagues at the University of California, Riverside, planted coast live oaks with symbiotic fungi in a three-chambered planter. The trees and fungi sat in one compartment, next to another that was walled off by two screens separated by an air space. The fungi grew into the second compartment, but the screen kept the oak roots out and prevented water from leaking in from the first compartment. The researchers then supplied fluorescently labeled water to a third compartment underneath, which contained the oaks' deep roots. The labeled water showed up in the fungi in both top compartments but not when researchers severed their connections with roots. This proves that the oaks were plumbing the depths to irrigate their fungal friends, the team reports online 18 October in the journal Oecologia.

Ecologist John Klironomos of the University of Guelph in Ontario, Canada, says this is the first study to show that plants supply water directly to fungi. This allows the fungi to survive and the plants to harvest nutrients that would otherwise be locked up in dry soil, away from the water needed to transport them. This could have wider ecological significance, he says: "This finding does promise to explain why plants are able to grow in habitats where nutrients and water are physically separated, such as in desert ecosystems."